1. Fast Neutron Radiographic and Tomographic Systems
A workable system for detecting explosives in airport luggage is urgently needed. A very small piece of modern explosive will destroy an airplane. These explosives are easy to hide and cannot be detected by current systems. For example, a plastic explosive hidden in a small radio apparently destroyed Pan American Flight 103 over Lockerbie, Scotland. The most accurate method would be a tomographic scan which could identify the elements which make up an explosive. For practical use in an airport, each scan would have to be completed in seconds. A system this advanced does not exist and is not possible under current technology.
Current methods for detecting explosives in airport luggage use uncharged particles such as X-rays and neutrons. X-rays are sensitive to differences in X-ray absorption coefficients in the luggage. However, explosives have absorption coefficients similar to many items commonly found in luggage. For this reason, detection systems using X-rays have high false alarm rates. X-ray computed tomography (CT) scanners are also used to inspect luggage. However, CT scanners are also sensitive to X-ray absorption coefficients, and so have the same problems as X-ray systems.
The most common nuclear based explosive detection methods are thermal neutron absorption (TNA) and n, gamma pulsed fast neutron spectroscopy. "Nuclear-Based Techniques for Explosive Detection", T. Gozani, R. Morgado, C. Seher, Journal of Energetic Materials, Vol. 4, pp. 377-414 (1986). The TNA detects the n, gamma reaction on nitrogen, and so searches only for nitrogen. "Airport Tests Of SAIC/FAA Explosive Detection System Based On Thermal Neutron Activation", P. Shea and T. Gozani, American Defense Preparedness Assoc. Proceedings, Cambridge, Mass., Oct. 26, 1988. TNA has an unacceptably high false alarm rate, since many materials other than explosives contain large amounts of nitrogen. Other problems with TNA include that the neutrons must be thermalized, the n, gamma cross section is in the millibarn range, it is difficult to obtain the spatial nitrogen concentration, and the background count is very high.
N, gamma pulsed spectroscopy detects the neutron inelastic scattered gamma rays from nitrogen, carbon, and oxygen. Zdzisaw Sawa and Tsahi Gozani, (PFNA Technique for the Detection of Explosives), Proc. of First Int. Sym. on Explosives Det. Technology, FAA Tech. Ctr., Atlantic City Int. Airport, N.J., Feb. 1992. Problems with n, gamma spectroscopy include that the cross sections are still in the millibarn range, the background counts are very high, determination of concentration as a function of position has large uncertainties, and it is difficult to make a gamma ray detector with adequate energy resolution and still maintain high count rate capability.
TNA and n, gamma spectroscopy search for explosives in an indirect way. Both cause a neutron interaction and then attempt to detect the resulting gamma rays. Neither method accurately pinpoints the location of an explosive in the luggage. A system is needed which can probe directly for explosives through first order interactions.
The most accurate method would be a tomographic scan which could identify the number densities of the elements which make up explosives in small volume increments through the luggage. Neutrons are an ideal probe, because neutrons interact directly with the atomic nuclie in the sample. A tomographic method which uses total cross sections rather than partial cross sections would be optimal. Current methods using neutron probes detect only second order effects such as gamma rays (partial cross sections) and do not provide tomographic images.
In conventional neutron tomography, neutrons are directed through a sample and the results are recorded on a detector. E. W. McFarland, R. C. Lanza and G. W. Poulos, "Multi-dimensional Neutron-computed Tomography Using Cooled, Charged-coupled Devices," IEEE Transactions on Nuclear Science, Vol. 38, No. 2, Apr. 1991. A number of runs at different angles are used to create a tomographic image. Conventional methods reconstruct the spatial distributions of macroscopic interaction cross sections from the attenuation of radiation passing through the sample.
These conventional methods use monoenergetic neutron probes. Monoenergetic probes show macroscopic cross-section variations, while providing little information regarding atomic or chemical structure. In contrast, a white neutron probe (fast neutrons of multiple energies) would provide information not possible from a monoenergetic neutron probe. Light provides a simple analogy. Under a red light (single energy), all objects appear as shades of red. However, a white light (multiple energies) reveals different colors and other details. A tomographic system using a white neutron probe would be far more advanced than any system in operation. Unfortunately, under current technology, it cannot be built. Problems include multiple scattering of radiation, inability to detect and measure the energies of fast neutrons over an x-y plane, and difficulty in reducing and analyzing data. These numerous problems would have to be solved in a single system.
Overly discussed that, under laboratory conditions, it is possible to identify different elements in a sample by using a white neutron beam. J. C. Overly, "Determinations of H, C, N and O content of Bulk Materials from Neutron Activation Measurements," Int J. Radiat Isot , Vol. 36, No. 3, pp. 185-191, 1985. Overly also discussed the scientific principle in deducing the number densities of elements along a neutron beam for a tomographic cut. "Element-Sensitive Computed Tomography With Fast Neutrons" by J. C. Overly, Nuc. Instr. and Meth. in Physics Research, B24/25 (1987) 1058-1062.
Overly required 23 hours for a single cut. A workable system for detecting explosives must reduce this time to seconds. Even after 23 hours, Overly obtained only an estimate of number densities at a single cut. A workable system must obtain a tomographic image of the number densities of atoms over the entire suitcase in a matter of seconds. As Overly acknowledged in his paper, current technology has not advanced to the point of applying these scientific principles in a workable system. The Federal Aviation Association ("FAA") recognized this critical need when requesting proposals for an invention using a white neutron probe for explosives. FAA "Guidelines for Preparing Responses to the Federal Aviation Administration's Broad Agency Announcement For Aviation Security Research Proposals,", p. 7, Nov. 1989. The FAA Guidelines acknowledge that the scientific principle has not been applied to airline security, even though it has been published for several years. In fact, the scientific principle has not been applied to any workable system. Neither the FAA, nor any other party, has found a way to address all of the problems which must be solved in order for a white neutron probe system to work. The following sections describe these problems.
2. Multiple-Scattering Correction of Radiation
Neutron tomographic systems must detect neutrons from the source while excluding neutrons scattered by the sample. If scattered neutrons reach the detectors, the tomographic image will not be accurate. X-ray systems use a detector located behind a shielded slot only a few millimeters wide. The slot is narrow enough to exclude most scattered radiation. This is one reason why a CAT scan takes so long. The detector row moves slowly across the body. A system using a large detector array could view the entire sample at once. However, such detectors (if they existed) could not be used for tomography until the multiple scattering problem is solved. The problem is even more critical when designing a system using a white neutron probe. The system must detect, measure the energies, and catalog the location of millions of neutrons passing through different parts of a sample each second. In order to provide meaningful data, the spatial resolution must be as small as several centimeters square. A workable system must eliminate multiple scattering while completing a scan in only a few seconds. There is a critical need for such a system.
Harding, Pat. No. 4,380,817 (1983) attempted to correct multiple scattering for purposes other than tomography. Harding's method measures electron density in a body through radiation which is "single scattered" from a narrow pencil beam of radiation. Harding's method shields the detector from the single scattered radiation, so that the detector measures only multiple scattered radiation. Then the multiple scattered radiation is subtracted from the sum of the single and multiple scattered radiation. This method is not workable for neutron tomography, which requires measuring neutrons which are not scattered (either single or multiple scattered). Harding's system would not work in a system using a white neutron probe for tomography.
3. X-Y Position Fast Neutron Detectors
A tomographic system using a white neutron probe must detect fast neutrons passing through a sample and pinpoint the location in the sample for each neutron. The system must perform these functions for millions of neutrons striking all portions of the sample each second.
One solution is to place the sample in front of an x-y detector which can record the two dimensional (x-y) coordinates of neutron interactions. The points of interaction on the x-y detector correspond to the locations in the sample. There are x-y neutron detectors, but most are for thermal neutrons. (Neutrons which have a kinetic energy of approximately 0.025 electron volts.) Many types of x-y detectors use an element that has a large fission cross section for thermal neutrons. The fission fragments are detected through the ionization they produce. McFarland, cited above, describes a detector using a sheet of .sup.6 Lif-ZnS. Lithium-6 has a large fission cross section for thermal neutrons. A fraction of the incident thermal neutrons interact with the Lithium-6 to produce Lithium-7. The Lithium-7 in turn fissions into a triton and an alpha particle, which cause a scintillation. A CCD camera records the scintillation and its position. Another variation uses an element that absorbs the thermal neutrons and emits X-rays or gamma rays.
The above types of detectors have a low detection efficiency for fast neutrons, since the fast neutron fission cross section is very small. Also, current x-y detectors cannot perform all of the functions needed for a tomographic system using a white neutron probe. These functions include measuring neutron energy, achieving high count rates, and collecting data in a way to facilitate tomographic imaging.
Fast neutron x-y detectors do exist, but have certain drawbacks. One type of x-y detector uses the multi-wire proportional counter with a proton radiator. B. Director, S. Kaplin and V. Perez-Mendez, "A Pressurized Multi-Wire Proportional Chamber for Neutron Imaging," IEEE Tr. on Nucl. Sc., Vol. NS-25, No. 1, Feb. 1978, 558-561. The proton radiator is a thin sheet of a hydrogen rich material such as polyethylene. A fraction of the incident neutrons scatter from the protons in the radiator. The resulting recoil protons enter the multi-wire proportional counter. The multi-wire proportional counter consists of thin gas filled cells with small wires running parallel throughout the cells. The wires are placed at high voltage. When a proton enters a cell close to a particular wire, a voltage pulse is created.
By recording the position of the voltage pulse, the position of the event is known in the direction perpendicular to the wires. By placing a second ionization chamber with wires running perpendicular to the first set of wires, the position in the other direction is known. The radiators must be very thin so that the recoil protons can escape. In order to achieve reasonable efficiency, many units must be placed in tandem. This setup cannot efficiently count neutrons below 3 MeV, since the radiator would require a width of nearly zero to allow the lower energy protons to reach the first cell. Efficiency would approach zero. These detectors do not allow measurement of neutron energy and would not provide the high count rates or neutron energy resolution required for a white neutron probe system.
An x-y detector for fast neutrons could be constructed from a large number of individual photomultiplier tubes. The applicant describes such a detector in "Contraband Detection Device", Ser. No. 07/635,996 filed on Dec. 31, 1990. One problem with this detector is that its electronics are very complex. A complete detector system is required for each photomultiplier tube. For example, a detector face of only 400 cm. by 400 cm., with a spatial resolution of 4 cm. by 4 cm., would require one hundred 4 cm. by 4 cm. detectors to form a single row 400 cm. long. One hundred rows of such detectors would provide a surface of 400 cm. by 400 cm. The system would require 10,000 individual detectors and 10,000 complete electronics systems. The system would be highly complex and expensive. Another problem is that the array is rigid and not capable of being geometrically configured for the optimal shape.
4. Collection, Reduction and Analysis of Data
Identifying contraband in sealed luggage is one of the most difficult tasks facing scientists. Explosives and drugs contain the same elements as most other items usually found in luggage. These elements include hydrogen (H), carbon (C), nitrogen (N) and oxygen (O). Explosives do contain characteristic ratios of these elements. However, in order to identify these ratios, a tomographic system would have to search an entire suitcase over small volume increments with an uncertainty of only a few percent. These tasks present enormous problems under current technology. Two problems, multiple scattering and measuring radiation over an x-y plane, were discussed above. In addition, the tomographic system would have to gather, reduce, and analyze data for millions of neutron interactions per second. The entire scan must be completed in seconds. The system must distinguish neutrons from gamma rays. New methods of data reduction and analysis would have to be developed for such a system to function with the speed and accuracy required for an airport system.
In summary, there are no tomographic systems which use a white neutron probe. There are numerous problems, unsolved under current technology, which prevent the operation of a workable system. These problems include multiple scattering of neutrons, detecting fast neutrons over a two-dimensional plane, calculating energies of the neutrons, and collecting, reducing and analyzing the data, all under circumstances when millions of neutrons will be incident on the sample and detector each second. The entire analysis of each sample must be completed in seconds. Such a system does not exist and cannot be built under current technology.